Make InstallObserver methods not pure-virtual

chrome/browser/extensions/install_observer.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_EXTENSIONS_INSTALL_OBSERVER_H_
 #define CHROME_BROWSER_EXTENSIONS_INSTALL_OBSERVER_H_
 
 #include <string>
 
 #include "ui/gfx/image/image_skia.h"
@@ skipping 13 matching lines @@
         bool is_platform_app);
 
     std::string extension_id;
     std::string extension_name;
     gfx::ImageSkia installing_icon;
     bool is_app;
     bool is_platform_app;
     bool is_ephemeral;
   };
 
-  virtual void OnBeginExtensionInstall(
-      const ExtensionInstallParams& params) = 0;
+  // Called at the beginning of the complete installation process, i.e., this
+  // is called before the extension download begins.
+  virtual void OnBeginExtensionInstall(const ExtensionInstallParams& params);
 
+  // Called whenever the extension download is updated.
+  // Note: Some extensions have multiple modules, so the percent included here
+  // is a simple calculation of:
+  // (finished_files * 100 + current_file_progress) / (total files * 100).
   virtual void OnDownloadProgress(const std::string& extension_id,
-                                  int percent_downloaded) = 0;
+                                  int percent_downloaded);
 
-  virtual void OnInstallFailure(const std::string& extension_id) = 0;
+  // Called if the extension fails to install.
+  virtual void OnInstallFailure(const std::string& extension_id);
 
-  virtual void OnExtensionInstalled(const Extension* extension) = 0;
-  virtual void OnExtensionLoaded(const Extension* extension) = 0;
-  virtual void OnExtensionUnloaded(const Extension* extension) = 0;
-  virtual void OnExtensionUninstalled(const Extension* extension) = 0;
-  virtual void OnAppsReordered() = 0;
-  virtual void OnAppInstalledToAppList(const std::string& extension_id) = 0;
+  // Called if the installation succeeds.
+  virtual void OnExtensionInstalled(const Extension* extension);
+
+  // Called when an extension is [Loaded, Unloaded, Uninstalled] or an app is
+  // installed to the app list. These are simply forwarded from the
+  // chrome::NOTIFICATIONs.
+  virtual void OnExtensionLoaded(const Extension* extension);
+  virtual void OnExtensionUnloaded(const Extension* extension);
+  virtual void OnExtensionUninstalled(const Extension* extension);
+  virtual void OnAppInstalledToAppList(const std::string& extension_id);
+
+  // Called when the app list is reordered.
+  virtual void OnAppsReordered();
 
   // Notifies observers that the observed object is going away.
-  virtual void OnShutdown() = 0;
+  virtual void OnShutdown();
 
  protected:
   virtual ~InstallObserver() {}
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_INSTALL_OBSERVER_H_